The present invention relates to apparatuses and methods for monitoring the condition of a tire, and, more particularly, to apparatuses and methods for wirelessly monitoring the condition of a tire, such as air pressure, to enable a driver to monitor that tire condition in a passenger compartment.
Wireless tire condition monitoring apparatuses have been proposed for enabling a driver of a vehicle to monitor the conditions of the tires in the passenger compartment. A typical wireless tire condition monitoring apparatus includes a plurality of transmitters and a receiver. Each of the transmitters is associated with a different one of the tires and is installed in the associated tire. The receiver is installed in the body frame of the vehicle. Each transmitter has a tire condition sensor that measures parameters indicating the condition of the associated tire, such as the air pressure and the temperature of the interior of the tire, and a transmitting circuit that wirelessly transmits data based on the measurement to the receiver. The receiver receives the data from the transmitter via a receiving antenna. The receiver then indicates the condition of the tire on a display, which is located, for example, near the driver seat in the passenger compartment.
In this apparatus, each transmitter transmits data indicating the condition of the associated tire at a constant bit rate, regardless of the operational mode of the vehicle, or regardless of whether the vehicle is moving or stopped. The battery of the vehicle powers the receiver. Thus, to save the battery power, a receiving circuit of the receiver is periodically turned on and off, instead of being maintained in a constantly turned-on state, when the engine is stopped. That is, the receiver operates intermittently as long as the engine is stopped.
The positions of the transmitters relative to the receiving antenna change in accordance with the movement of the tires. Thus, when one of the transmitters transmits a radio wave to the receiver, the level of voltage induced in the receiving antenna by the radio wave changes in relation to the rotational angle of the associated tire, with reference to FIG. 4. The change of the induced voltage level (hereafter, xe2x80x9can induced voltage patternxe2x80x9d) is determined by factors such as the type of the vehicle, the positions of the tires, the types of the tires and wheels, and the location of the receiving antenna. The induced voltage pattern includes an induced voltage area in which the induced voltage dose not reach a level corresponding to a minimum receiving sensitivity of the receiver (hereafter, xe2x80x9ca null pointxe2x80x9d).
That is, the receiver accurately receives data from the transmitters only when the induced voltage level does not correspond to the null point, or is in a receivable induced voltage area. Further, if the rotational speed of the tires is increased due to acceleration of the vehicle, the time for which the induced voltage level remains in the receivable induced voltage area is shortened. That is, the higher the rotational speed of the tires becomes, the more difficult it becomes for the receiver to accurately receive data from the transmitters. In other words, if the data transmitting time exceeds the time for which the induced voltage level remains in the receivable induced voltage area, the receiver cannot completely receive the data from the transmitters.
To solve this problem, for example, the transmitting power of each transmitter may be raised to eliminate the null point. However, the transmitting power is restricted by radio wave regulations. Further, to prolong the lives of the batteries that power the transmitters, it is undesirable to raise the transmitting power of the transmitters. It is thus impossible to raise the transmitting power of each transmitter to a level sufficient for eliminating the null point.
Alternatively, the null point may be eliminated by optimizing the location of the receiving antenna. In this case, the induced voltage pattern of the receiving antenna is affected by factors such as the type of the vehicle, the positions of the tires, and the types of the tires and wheels. That is, the optimization of the location of the receiving antenna needs a large-scale evaluation and is thus infeasible. Further, the design of the vehicle may make it impossible to install the receiving antenna at an optimal location. In addition, it is desirable that the location of the receiving antenna be selected freely. It is thus impossible to eliminate the null point by optimizing the location of the receiving antenna.
It is also possible to shorten the data transmitting time of each transmitter to reliably transmit data even when the vehicle traveling speed is relatively high. The data transmitting time may be shortened by decreasing the bit count (the data length) of transmitted data or increasing the bit rate of the transmitted data. However, since the above-described apparatus employs a minimum bit count for transmitted data, the bit count of the transmitted data cannot be further decreased. Thus, the data transmitting time must be shortened by increasing the bit rate of the transmitted data. In this case, the increased bit rate must be selected to sufficiently shorten the data transmitting time, compared to the time for which the induced voltage level remains in the receivable induced voltage area, even if the induced voltage pattern includes the null point. Further, data transmission can be further stabilized by repeating the data transmission several times in a single transmission cycle, in addition to the increasing of the bit rate.
However, as described, the receiver operates intermittently to save the battery power when the engine is stopped. Thus, if the data is transmitted at a relatively high bit rate in a relatively short time when the engine is stopped, the receiver cannot stably receive the transmitted data unless the receiving circuit is turned off only for a relatively short time, or unless the receiving circuit remains turned on for a sufficiently long time, during the intermittent operation. In contrast, to save the battery power when the engine is stopped, the receiver needs to be turned on for a relatively short time during the intermittent operation. It is thus impossible to prolong the time for which the receiving circuit remains turned on and save the battery power at the same time.
Accordingly, it is an objective of the present invention to provide an apparatus and a method for monitoring the condition of a tire that transmit data at an optimal bit rate regardless of the operational mode of a vehicle, or regardless of whether the vehicle is moving or stopped.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, the invention provides a transmitter of an apparatus for monitoring the condition of a tire attached to a vehicle. The transmitter includes a tire condition sensor, a transmitting circuit and a controller. The tire condition sensor measures a parameter indicting the condition of the tire. The transmitting circuit wirelessly transmits, at a variable bit rate, data representing the measured parameter. The controller controls the variable bit rate of the data transmitted by the transmitting circuit.
The present invention also provides a method for monitoring the condition of a tire attached to a vehicle. The method includes the steps of measuring a parameter indicating the condition of the tire, wirelessly transmitting, at a variable bit rate, data representing the measured parameter, and varying the variable bit rate of the transmitted data in accordance with a traveling speed of the vehicle.
The present invention further provides a method for monitoring the condition of a tire attached to a vehicle. The method includes the steps of measuring a parameter indicating the condition of the tire, and wirelessly transmitting data representing the measured parameter at more than one bit rate.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.